Subcritical Carbon Dioxide Dehumidifier

ABSTRACT

A subcritical carbon dioxide dehumidifier having a compressor, a condenser, a receiver, an expansion valve, an evaporator, and an expansion tank and using carbon dioxide as a refrigerant.

FIELD OF THE INVENTION

The present invention relates to dehumidifiers, in particular, to dehumidifiers that use carbon dioxide as a refrigerant.

BACKGROUND

The use of carbon dioxide as a refrigerant is well known, however, the fact that the critical point of carbon dioxide is relatively low, particularly compared to many other common refrigerants, creates challenges for its successful application. Carbon dioxide has a critical point of 31.1° C. and 1,071 psia, above which it behaves as a supercritical fluid. Normal ambient temperatures can exceed the critical temperature of carbon dioxide, resulting in high operating pressures within a dehumidifier system.

As a result, dehumidifiers using carbon dioxide as a refrigerant have hereto been designed and built to withstand pressures of up to 1,300 psia. This increases the cost of such units to the point where they are commercially uncompetitive with dehumidifiers that use other refrigerants, such as R-404a or R-407c. However, carbon dioxide has a number of desirable characteristics as a refrigerant. For example, many common refrigerants, such as R-407c, are hazardous if inhaled, whereas carbon dioxide is non-toxic. Carbon dioxide is also a by-product of many industrial processes and, as a result, is readily available at a low cost. Further, refrigerants such as hydrofluorocarbons (HFCs) can have a serious environmental impact, through their high global warming potential (GWP).

In applications where ambient temperature in the operating environment generally remains below the critical temperature for carbon dioxide, such as in a hockey arena, dehumidifiers that use carbon dioxide as a refrigerant may be designed and built having lower operating pressures. However, during a power outage or when the seasonal ambient temperatures are above the critical temperature and pressure within the dehumidifier system can increase above the critical point, the excess pressure created by the supercritical carbon dioxide must be released by discharging a portion of the refrigerant. The system will then require a new charge of refrigerant before it can return to normal operation, which can cause delays and be costly for large dehumidifiers, such as those used in hockey arenas.

SUMMARY OF THE INVENTION

A subcritical carbon dioxide dehumidifier, according to the present invention, has a compressor, a condenser, a receiver, an expansion valve, an evaporator, and an expansion tank and uses carbon dioxide as a refrigerant.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more clearly understood, a preferred embodiment thereof will now be described in detail by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic view of a subcritical carbon dioxide dehumidifier, according to the present invention.

FIG. 2 is graph showing the compressor operating envelope for the subcritical carbon dioxide dehumidifier.

DESCRIPTION OF THE INVENTION

A subcritical carbon dioxide dehumidifier, according to the present invention, uses carbon dioxide as the refrigerant and maintains the system at a subcritical temperature throughout the refrigeration cycle. The dehumidifier is described herein with reference to the example application of a hockey arena, but may be used in any application where the ambient temperature is generally below the critical temperature of carbon dioxide. The dehumidifier has an expansion tank to accommodate an increase in pressure resulting from the carbon dioxide refrigerant temperature increasing above its critical point, such as during a power outage or seasonally. This permits the system to avoid discharging refrigerant to relieve excess pressure and thereby retain a full refrigerant charge when the ambient temperature is in the supercritical range of carbon dioxide.

As shown schematically in FIG. 1, the carbon dioxide dehumidifier, referred to herein as the dehumidifier, has a compressor 1, a condenser 2, a receiver 3, an expansion valve 4, an evaporator 5, and an expansion tank 6. The operation of the compressor 1, condenser 2, receiver 3, expansion valve 4, and evaporator 5 in the basic closed circuit refrigeration cycle are well known and, accordingly, are described briefly herein. These parts are located within a housing (not shown), having an air inlet and outlet, which is generally located on the roof of the facility and the air is ducted to and from the dehumidifier. The dehumidifier may also be located directly in the space it is conditioning, such as on a mezzanine of a hockey arena, and the air inlet receives air from the space at one location and the air outlet blows air back into the same space at a different location.

The basic closed circuit refrigeration cycle of the dehumidifier operates as follows. Low-pressure low-temperature carbon dioxide gas is drawn into the compressor 1 and is compressed and supplied to the condenser 2 as a high-pressure high-temperature gas. In the condenser 2, the high-pressure high-temperature carbon dioxide gas transfers heat to the air passing over the condenser 2 and condenses to high-pressure high-temperature liquid carbon dioxide. The high-pressure high-temperature liquid carbon dioxide then enters the receiver 3, where it is stored until needed. The high-pressure high-temperature liquid carbon dioxide then flows through the expansion valve 4, which restricts the flow of liquid and lowers the pressure of the liquid. The low-pressure low-temperature liquid carbon dioxide then enters the evaporator 5, typically as an aerosolized mixture of gas and liquid, where it absorbs heat from the air. This causes the liquid carbon dioxide to evaporate as it absorbs heat in the evaporator 5, to result in a low-temperature low-pressure gas that is supplied to the compressor 1 to continue the refrigeration cycle.

The above refrigeration cycle is carried out in the main line 7 of the dehumidifier, which is defined by the compressor 1, the condenser 2, the receiver 3, the expansion valve 4, the evaporator 5, and the connections therebetween. The main line 7 of the dehumidifier has a high-pressure side 9, between the compressor 1 and the expansion valve 4, and a low-pressure side 10, between the expansion valve 4 and the compressor 1.

As shown in FIG. 1, the expansion tank 6 is on a secondary line 8, which is connected to the main line 7, and together with the main line 7 forms a closed refrigeration circuit. Preferably, the expansion tank 6 connects to the main line 7 in two places, a first connection 11 on the high-pressure side 9, between the evaporator 5 and the compressor 1, and a second connection 12 on the low-pressure side 10, between the condenser 2 and the receiver 3. This permits the first connection 11 to operate as a drain line from the expansion tank 6 to the compressor suction line.

A control valve 13 on the first connection 11 prevents the flow of high-pressure liquid carbon dioxide into the expansion tank 6 under normal operating conditions. Preferably, the control valve 13 is a normally open solenoid valve, so that if the control valve 13 is not energized, for any reason, the solenoid valve opens and allows the high pressure gas to expand into the expansion tank 6. The control valve 13 is opened when pressure in the high-pressure side 9 increases beyond a safety threshold value, to permit high-pressure carbon dioxide to flow into the expansion tank 6 and thereby relieve pressure from the main line 7.

Preferably, the main line 7 is provided with one or more pressure relief valves 14, which operate as a failsafe to release pressure in the event the control valve 13 of expansion tank 6 fails and pressure builds up in the main line 7. The expansion tank 6 may also have a pressure relief valve 14, in case the pressure in the expansion tank 6 exceeds acceptable levels. Because carbon dioxide is used as the refrigerant, there is minimal safety hazard in venting excess refrigerant through the pressure relief valves 14. Nonetheless, preferably, the pressure relief valves 14 discharge excess refrigerant to an outside area.

A filter 15 may be included on the main line 7 between the receiver 3 and the expansion valve 4 to maintain the clean and moisture free operation of the dehumidifier. A view port 16 may also be provided on the main line 7 between the receiver 3 and the expansion valve 4 to permit observation of the liquid column passing between the receiver 3 and the expansion valve 4.

A control system monitors temperatures and pressures throughout the dehumidifier and controls the operation of the dehumidifier. As shown in FIG. 1, the control system monitors the temperature and pressure in the main line 7 between the compressor 1 and the condenser 2 and between the receiver 3 and the expansion valve 4. The control system may also monitor the humidity and temperature of the space and control the operation of the dehumidifier based on set humidity and/or temperature thresholds.

In a typical hockey arena, the ambient temperature is roughly 6° C. Accordingly, the air passing over the condenser 2 cools the high-pressure carbon dioxide vapour into a high-pressure liquid and at the same time increases the temperature of the air to approximately 10° C. The air then passes through the evaporator 5, where the temperature is below the dew point of the air, thereby dehumidifying the air and cooling it to approximately 7° C. The net increase in temperature of the air passing through the dehumidifier is desirable for applications such as hockey arenas, because it reduces the amount of heating otherwise required for the space.

At normal operating conditions, the high side pressure is between 450 psi and 650 psi, while the low side pressure is between 300 psi and 400 psi. High side pressure refers to the pressure of the carbon dioxide in the high-pressure side 9 of the main line 7. Similarly, low side pressure refers to the pressure of the carbon dioxide in the low-pressure side 10 of the main line 7.

The present invention has been described and illustrated with reference to an exemplary embodiment, however, it will be understood by those skilled in the art that various changed may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention as set out herein. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed herein. 

What is claimed is:
 1. A subcritical carbon dioxide dehumidifier, comprising a compressor, a condenser, a receiver, an expansion valve, an evaporator, and an expansion tank and connections therebetween forming a closed refrigeration circuit.
 2. The subcritical carbon dioxide dehumidifier of claim 1, wherein the compressor, condenser, receiver, expansion valve, evaporator, and the connections therebetween define a main line.
 3. The subcritical carbon dioxide dehumidifier of claim 2, wherein the main line has a high-pressure side between the compressor and the expansion valve and a low-pressure side between the expansion valve and the compressor.
 4. The subcritical carbon dioxide dehumidifier of claim 3, having a secondary line that connects to the main line by a first connection and a second connection, and wherein the expansion tank is located on the secondary line.
 5. The subcritical carbon dioxide dehumidifier of claim 4, wherein the first connection connects to the high-pressure side of the main line and the second connection connects to the low-pressure side of the main line.
 6. The subcritical carbon dioxide dehumidifier of claim 5, wherein the first connection connects between the evaporator and the compressor.
 7. The subcritical carbon dioxide dehumidifier of claim 6, wherein the second connection connects between the condenser and the receiver.
 8. The subcritical carbon dioxide dehumidifier of claim 7, having a control valve on the first connection that selectively prevents the flow of carbon dioxide into the expansion tank.
 9. The subcritical carbon dioxide dehumidifier of claim 8, wherein the control valve is a normally open solenoid valve.
 10. The subcritical carbon dioxide dehumidifier of claim 9, having one or more pressure relief valves on the main line.
 11. The subcritical carbon dioxide dehumidifier of claim 10, having a pressure relief valve on the expansion tank.
 12. The subcritical carbon dioxide dehumidifier of claim 11, having a filter on the main line, located between the receiver and the expansion valve. 